The present invention relates to an iron electrode for chemoelectric cells which electrode has been stabilized by the addition of a small amount of a lead sulfide. The invention also relates to chemoelectric cells containing the stabilized iron electrode.
The iron electrode is beginning a renaissance for use as negative electrode material for alkaline accumulators. It has been shown that the iron electrode is a useful anode for use in metal-air batteries. Iron has shown a number of advantageous qualities including low price, its nonpoisonous character compared to cadmium and also a potentially high capacity density. Iron, however, also exhibits several disadvantageous properties, for instance, self discharge and poor capacity data at low temperatures.
Iron exhibits, contrary to cadmium and zinc, which are also used as anode metals in chemoelectric cells, two discharge steps, namely a first step from the valance 0 (Fe) to the valance 2 Fe.sup.++) and a second step from the valance 2 (Fe.sup.++) to the valance 3 (Fe.sup.+++). The first discharge step is generally the only step utilized whereas the second discharge step may serve as spare capacity. The ideal discharge curve for an iron electrode in general exhibits the appearance shown in FIG. 1 which is a plot of the electrode potential of an iron anode versus a mercury oxide reference electrode. However, the discharge pattern may sometimes be disturbed which produces a lower capacity during the first discharge step and a larger capacity in the second discharge step than normal. This is shown, in principle, in FIG. 2. It is not established what causes this redistribution of the discharge pattern. One possible explanation is that the structure of the porous iron electrode material is disturbed or "partially limited". That is, during the first discharge step (from 0 valence to plus 2 valence), the porosity of the electrode is reduced since the reaction product Fe(OH).sub.2 take up a larger volume than the reacted iron metal (Fe). With a partially limited electrode structure, the pores for mass transfer of ions and electrolyte may be plugged up for instance at the surface of the electrode layer. During the second discharge step (from plus 2 valence to plus 3 valence), the volume of the solid electrode materials is reduced (since Fe(OH).sub.3 has a lower volume than Fe(OH).sub.2) which opens up the structure. Discharge to the second step again opens up the structure which makes the inner part of the electrode available again for the electrochemical reaction. This allows discharge at a lower potential than with the normal discharge pattern. This phenomenum which gives the user an impression of a certain capriocity of the iron electrode is an important drawback to its practical use in a battery. One tries in general to reduce the effects of this phenomenum by a slow charge followed by a deep discharge so as to restructure the electrode. The purpose of the present invention is to eliminate the disturbances in the discharge pattern to provide the ideal discharge mode for iron electrodes.
It is known since the beginning of akaline accumulator technology that addition of sulfide as iron sulfide and in quantities of above 0.2 to 0.3% by weight, in general much more (e.g., 4 or 5%), exhibits a useful effect on the capacity density and reduces the self discharge of the iron electrode. The addition of iron sulfide, the addition of a sulfide to the electrolyte or the addition of sulphur compounds to the electrode material is described for instance, in Swedish Pat. No. 196,168 and in the German Offenlegungsschrift No. 2,206,828.
Addition of these sulfide and sulfur compounds and in these quantities has, however, not been found to eliminate the variability effect of the iron electrode discharge from the ideal described above.
It is also known that addition of lead sulfide to zinc electrodes improves the charge efficiency of the zinc electrodes due to the influence of the lead sulfide on the hydrogen overvoltage. However, the lead sulfide addition could possibly not influence the discharge mode of the zinc electrode since zinc has only one discharge step. Furthermore, the lead sulfide could not have an influence on the electrode structure or porosity since the zinc electrode goes at least partially into solution during discharge as zincate and the structure is at least partially rebuilt during each charge and discharge cycle. Lead sulfide therefore exhibits a quite different technical effect when it is used as addition to the zinc electrode. The addition of lead sulfide to iron electrodes has not been suggested or tried before because iron sulfide exhibits about the same useful effect on the hydrogen overvoltage as such and it has therefore not been of any interest to add lead sulfide for this purpose.
It is an object of this invention to provide an iron electrode which substantially alleviates the problems of the prior art.
It is also an object of this invention to provide an iron electrode for use in a chemoelectric cell which shows a substantially uniform discharge pattern.
It is further an object of this invention to provide an improved chemoelectric cell containing an iron electrode which shows a substantially uniform discharge pattern.